1. Field of the Invention
The present invention relates to a configuration of an optical output control circuit or an automatic power control circuit suitable for use with an optical transmitter, etc. in an optical communications system.
2. Background Art
FIG. 8 shows a conventional general optical output control circuit or automatic power control circuit (hereinafter referred to as an APC circuit as necessary). The APC circuit comprises: an LD module including a laser diode LD and a monitor photodiode PD; a current-to-voltage conversion circuit 2 (APC-IV conversion circuit) for converting the output current of the monitor photodiode PD into a voltage level; an optical output power control circuit 4 for performing control so as to eliminate the difference between the output voltage V2 (feedback voltage) of the current-to-voltage conversion circuit 2 and the LD optical output power setting voltage V1 output from an optical output power setting input circuit 12; a low-pass filter 6 (LBW-LPF) for deciding the loop band width of the APC circuit; an LD drive circuit 8 (LD-Drv circuit) for controlling the drive current of the laser diode LD; an LD drive setting input circuit 18 (LD-Drv setting input circuit) for deciding the LD drive setting input; low-pass filters 16 and 22 (APC-LPF and ACC-LPF respectively) connected so that the input voltages to the optical output power control circuit 4 and the LD drive circuit 8 change slowly.
Though not shown, the LD drive circuit 8 comprises a circuit for driving the bias current of the laser diode LD and a circuit for driving the modulation current of the laser diode LD. Furthermore, the LD module comprises a thermistor and a thermal electric cooler (TEC) made up of a thermoelectric conversion element. The temperature of the LD is kept constant by an automatic temperature control circuit (ATC circuit) not shown.
The APC circuit detects the backlight, etc. of the laser diode LD by use of the monitor photodiode PD, feeds back the detected optical output power to itself to control the drive current of the laser diode LD, and thereby controls the optical output power of the laser diode LD so as to keep it constant. The operation of the APC circuit will be described in detail. The optical output power of the laser diode LD is determined by detecting the backlight, etc. of the laser diode LD by use of the monitor photodiode PD. The monitor photodiode PD outputs an optical current corresponding to the optical output power of the laser diode LD, and the output optical current is converted into voltage information by the current-to-voltage conversion circuit 2. The voltage information is fed back to the optical output power control circuit 4.
The optical output power control circuit 4 detects and amplifies the difference between the LD optical output power setting voltage VI and the feedback voltage V2 and inputs it to the LD drive circuit 8. The LD drive circuit 8 for driving the modulation current of the laser diode LD controls the LD drive current based on the input voltage from the optical output power control circuit 4, keeping the LD optical output power constant. Furthermore, the low-pass filter 6 (LBW-LPF) is inserted in the APC feedback loop to remove high-frequency signals included in the optical current of the monitor photodiode PD, thereby cutting off signals (noise) at high frequencies.
It should be noted that the APC circuit may include an optical output cutoff circuit 10 for cutting off the optical output of the laser diode LD according to external optical output cutoff signal input. In this case, it is possible to cut off the optical output of the laser diode LD by inputting an external optical output cutoff signal to the optical output cutoff control circuit 10 during the time when the circuit operation is unstable after power-on or even in the ordinary operation in which the optical output power of the laser diode LD is normally controlled so as to be kept constant. Then, when the optical output cutoff signal input has been removed, the APC circuit is restored to the normal operation and therefore the optical output power of the laser diode LD is controlled so as to be kept constant.
It should be further noted that the APC circuit may include the low-pass filter 16 (APC-LPF) between the optical output power setting input circuit 12 and the optical output power control circuit 4, and the low-pass filter 22 (ACC-LPF) between the LD drive setting input circuit 18 and the LD drive circuit 8. In this case, the output voltages of the optical output power setting input circuit 12 and the LD drive setting input circuit 18 slowly increase after they are passed through the low-pass filter 16 (APC-LPF) and the low-pass filter 22 (ACC-LPF), respectively, according to their time constants. Therefore, the ordinary operation of the APC circuit can be started when the APC circuit has assumed its stable operational state after power-on. Furthermore, since the setting voltages determined based on the time constants of the low-pass filters 16 and 22 are input to the APC circuit, overshoot of the optical output does not occur.
However, in an APC circuit having the optical output cutoff function described above, an overshoot case occurs in which the optical output power of the laser diode LD exceeds its set value in transient response when the optical output cutoff input has been removed.
FIG. 9 is a signal waveform diagram used to describe the operation of the APC circuit shown in FIG. 8. Specifically, this diagram shows the optical output power obtained when the optical output cutoff signal is input or removed while the APC circuit is controlling the LD optical output power so as to keep it constant. Referring to FIGS. 8 and 9, reference numeral V1 denotes an LD optical output power setting voltage, while reference numeral V2 denotes the feedback voltage from the current-to-voltage conversion circuit 2. If the High level is input as the optical output cutoff signal, the optical output cutoff control circuit 10 performs control such that the drive current of the laser diode LD is reduced to 0 mA.
If an optical output cutoff signal having a high-speed pulse waveform is input as shown in FIG. 9, the feedback voltage V2 is reduced to 0 V because the optical output of the laser diode LD is cut off. At that time, since the LD optical output power setting voltage V1 continues to be input to the optical output power control circuit 4, the output voltage of the optical output power control circuit 4 is at its maximum. Therefore, when the optical output cutoff signal input has been removed and thereby the optical output cutoff control circuit 10 performs control such that the drive current of the laser diode LD can be driven, the output of the optical output power control circuit 4 is in excess until the feedback loop of the APC responds, generating overshoot as shown in FIG. 9. Overshoot of the optical output causes destruction of the LD.
Accordingly, an object of the present invention is to provide an optical output control circuit or an APC circuit capable of reducing overshoot of the optical output power of a light emitting device such as a laser diode (LD) when the optical output is cut off or the cutoff of the optical output is cancelled.
According to one aspect of the present invention, an optical output control circuit comprises a light emitting device. A drive circuit will output a drive signal to the light-emitting device so that the light emitting device emits light. A drive setting input circuit will output to the drive circuit a drive setting signal to keep the light emitting device in a constant state. An input circuit for optical output power setting will outputs an optical output power setting signal to set optical output power of the light-emitting device. A monitor circuit will monitor the optical output power of the light emitting device, and will output a monitor signal. A control circuit for optical output power will compare the optical output power setting signal with the monitor signal, and will input a comparison signal to the drive circuit to control the drive signal of the drive circuit. A first control circuit for optical output cutoff will receive an optical output cutoff signal for cutting off optical output of the light emitting device, and will stop the drive circuit from outputting the drive signal while the optical output cutoff signal is being input. A second control circuit for optical output cutoff will receive the optical output cutoff signal, and will change the optical output power setting signal, being input to the optical output power control circuit, to an optical output cutoff level while the optical output cutoff signal is being input. Thus, the optical output power of the light emitting device is controlled.
In another aspect of the present invention, in the optical output control circuit, preferably a third control circuit for optical output cutoff will receive the optical output cutoff signal, and will change the drive setting signal, being input to said drive circuit, to a level to reset an operation state of the light emitting device while the optical output cutoff signal is being input.
In another aspect, in the modification of the optical output control circuit, the first control circuit for optical output cutoff may not be included. In addition, the second control circuit for optical output cutoff may not be included further.
Other features and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawings.